Literally, when there are "smoldering embers," there is no longer any flame. The fire appears to be out. However, there are red-hot embers. The first is not really out. A puff of air blowing on the embers, or stirring them with a stick or poker, cause make the burst in to flame and re-start the fire.
In this context, "smoldering embers" mean that the culture war seems to have subsided, but is not really over, and could be rekindled to flame again.